(1) Field of the Invention
The invention relates to the fabrication of integrated circuit devices, and more particularly, to a new method for cleaning phase shifting masks.
(2) Description of the Prior Art
Photomasks, which are a key component in the performance of photolithographic exposures, are used repeatedly and cyclically during the process of creating semiconductor devices. A photomask contains patterns that are projected from the mask to underlying layers of semiconductor material, such as a layer of photoresist. The standard mask comprises a transparent substrate on the surface of which a patterned layer of opaque material has been created. Typically used for the opaque material is chromium that has been deposited over the quartz substrate to a thickness of about 1,000 Angstrom. Alternate opaque materials for the creation of the patterned layer on the surface of a photolithographic mask are nickel and aluminum while for more sophisticated masks such as halftone phase shift masks MoSiON is used as the phase shifter material. For the substrate most typically is used quartz whereby however glass and sapphire can also be used for this purpose. Phase shift masks apply the principle of phase shifting of the light as the light passes through the mask. Phase shift masks are used for the creation of device features of sub-micron dimensions, where projected light that is in extreme close proximity mutually influences adjacent light, having a detrimental effect on the formation of the exposed patter. A further level of sophistication is introduced by the use of regions on the surface of the substrate of the photomask that pass light in a graded manner, the so-called Gray-Tone Masks (GTM). Light passing characteristics of the GTM mask can be adjusted so that not only complete passing or complete blocking of light takes place but so that the mask provides a graded exposure that may for instance be if use in creating dual damascene structures, where depth of light exposure can be used for non-uniform removal of a layer of photoresist over the thickness of the layer of photoresist.
Due to the nature of the repeated and cyclical use of a photomask, residue of semiconductor materials such as metal and the like frequently accumulates on the surface of the mask. The mask must therefore, after the mask has been used for a certain number of photolithographic exposures, be cleaned in order to remove the accumulated deposits which otherwise interfere with and inhibit the desired performance of the mask. The mask is therefore removed from the exposure tool and typically chemically cleaned.
The phase shift mask provides a solution to the problems that are associated with creating devices features of sub-micron dimensions, which pose one of the most challenging aspects of photolithographic exposures of semiconductor surfaces with the objective of creating patterns therein. A typical halftone phase shift mask uses a patterned and etched layer of MoSiON as the phase shifter. The pattern of MoSiON is known to have good phase shifter characteristics but as a disadvantage of MoSiON must be cited that MoSiON is very sensitive to chemical cleaning. This requires that an optimum process is provided for cleaning the surface of a halftone phase shift mask that uses a patterned and etched layer of MoSiON as the phase shifter. The invention provides such a process, which is particularly adapted to taking into consideration effects of pellicle removal and the removal of residues that remain on the surface of the mask after the pellicle removal has been completed.
U.S. Pat. No. 5,152,878 (Datta et al.) shows an electrochemical mask cleaning process.
U.S. Pat. No. 5,578,273 (Hanson et al.) and U.S. Pat. No. 5,472,516 (Lunch) reveal related SC1 cleaning processes for photomasks.